add ebuilds for dev-db/etcd* packages and updates for app-admin/fleet package

I haven't touched src_compile() at all and didn't know much about the build system at the point of creation.
The coreos-overlay also has an ebuild for etcd. Their compile and install looks like this:
```
src_compile() {
./build
}
src_install() {
dobin ${S}/bin/${PN}
systemd_dounit "${FILESDIR}"/${PN}.service
systemd_dotmpfilesd "${FILESDIR}"/${PN}.conf
}
```
They also skip the whole man-page generation. Should we skip that too and instead install the original documentation files using dodoc?